[CodeGen] Removes unwanted optimisation for TargetConstantFP

This 'FIXME' popped up in the development of an out-of-tree backend.
Quick fix, but first llvm upstream patch, therefore I do not have commit rights, so if approved please commit?

- Test is not included as this came up in an out-of-tree backend (if required, please hint on how to test this).

Patch by simveg (Simon)

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D93219